Hand tool drivers are well-known. Although drills are the most common tools used with such drivers, the tools may also comprise screwdrivers, nut drivers, burrs, mounted grinding stones, and other cutting or abrading tools. Since the tools may have shanks of varying diameter or may have a polygonal cross-section, the device is usually provided with a chuck that is adjustable over a relatively wide range. The chuck may be attached to the driver by a threaded or tapered bore.
Numerous chucks have been developed in the art. Traditionally, chucks have three jaws spaced circumferentially approximately 120° apart from each other are constrained by angularly disposed passageways in a body attached to the driveshaft. The chuck is configured so that rotation of the body in one direction with respect to a constrained nut forces the jaws into or away from a gripping relationship with a tool shank. Such a chuck may be keyless if it can be tightened or loosened by manual rotation from a fully opened position to a position where the jaw elements contact each other.
Unfortunately, when the jaws are in their substantially fully retracted position, bits of less than a fixed diameter can be inserted improperly between the jaw elements. Under these conditions, when the jaws are tightened, the bit can be engaged so that it is not axially aligned with the drive mechanism. Traditionally, a user is, therefore, forced to hold the bit in one hand to manually center the bit within the axial bore.